How often is there an option to upgrade the palace?

[Blanchflower]

OK, I have been wondering about this for some time. What should one do for the option to upgrade the palace pops up? Is this random? How can I increase the frequency for this?

Thanks!

 

[nyarmondt]

Well, I guess it's related with some events occuring during your reign:
The first successful defense against barbarians
The first successful overtake of a city
The first wonder you build
The first time you reach a certain amount of citizens
The first time you change government
...

I'm not totally sure though. These are more or less educated guesses of my part.
Mind you, that if you play a mod, there's no palace build screen.

 

[Bartleby]

It was discussed to an extent here quite recently.